Filtration filters are frequently used when purifying service water by removing contaminants or impurities from, for example, waste water of a sewage treatment plant which treats sewage such as factory waste water or living drainage, or when purifying fresh water by removing, for example, salt from seawater. Such filtration filters are known to be made up of a polymer material such as, for example, reverse osmosis membranes using a polymer membrane of methyl acetate. The reverse osmosis membranes have countless through holes with a diameter of several nanometers. When sewage or seawater is pressed and passed through the reverse osmosis membranes, water molecules pass the through holes in which the diameter of one water molecule is about 0.38 nm. However, molecules of contaminants having a size of dozens of nanometers or sodium ions around which water molecules are coordinated due to hydration cannot pass the through holes. As a result, the reverse osmosis membranes purify service water or fresh water from the sewage or seawater by separating the water molecules from the contaminants or sodium ions.
However, when service water is purified from waste water using such reverse osmosis membranes in a developing country or a distressed area with a natural disaster, bacteria in the waste water decompose the polymer membranes. Thus, there is a problem in that the lifespan of the reverse osmosis membranes are extremely shortened.
In addition, since the reverse osmosis membranes use a polymer membrane as a principal constituent element, the reverse osmosis membranes are poor in strength. Thus, there is a problem in that, when the pressure applied to sewage or seawater (primary side pressure) is increased in order to enhance the purification efficiency, the reverse osmosis membranes may be torn.
Thus, filtration filters in which a zeolite membrane for separation which is stable without being decomposed by bacteria is used have been recently used. See, for example, Patent Document 1 and Patent Document 2.